The DPD (N,N-diethyl-p-phenylenediamine) method for testing chlorine levels was introduced in 1957, and has become the most widely used method for determining free and total chlorine in water. The test has always been used as a liquid test and not developed as a reagent strip for dipping and reading the color by comparing it to a color chart. Current tests in the market that are called DPD strip tests are merely transfer tests not for dip-and-read.
Two standard DPD colorimetric methods are generally recognized in the international community and approved by most states for testing pools and spas. DPD test methods are based on liquid test kits that involve mixing a sample of pool water with chemicals dispensed from a dropper bottle and reading the color developed with a photometer. Those that are currently called DPD test strips have DPD powder attached to a paper or plastic strip/stick. The strip is dipped in a tube of water collected from a pool or spa allowing the powder to dissolve in the water sample and react with any chlorine to give a pink color. The developed color is then read by comparison to a color chart, or on a colorimeter or photometer type device. In essence, the strip-type device is merely a transfer agent for the DPD powder. Each of the U.S. Pat. Nos. 7,491,546; 7,333,194; and, 6,004,820 disclose delivery-type devices, not dip-and-read test strips.
Some prior art (U.S. Pat. Nos. 3,937,613 and 4,275,031) describe reagent delivery devices that include inert plastic strips that are rigid enough to release the reagents by stirring the stick and reading the pink color with a photometer. In U.S. Pat. No. 4,275,031, Fisher et al. describe the use of polyvinyl alcohol (PVA) as an embedding polymer for DPD sulfate and waiting for ten minutes instead of immediate reading. Supposedly, the PVA dissolves in the sample and releases the DPD sulfate suggesting the PVA is an embedding polymer that retards the release of the DPD sulfate. Again, the color developed is read by use of a hand-held color reading device. The transfer agent can be of variable thickness or texture, but it must be able to deliver enough analytical reagent for immediate photometric analysis of free chlorine with accuracy. What is needed is a more stable formulation of DPD for use on test strips.